I have quite a few cases of assorted sizes that I've picked up hither and yon, and a lot of them are very heavily tarnished to a dark gray, almost black, from sitting in the dirt/weather for years. Can anybody recommend a safe chemical dip of some kind that I can soak these in to remove the tarnishing before tumbling? Something that won't hurt the brass itself or weaken it. I can probably beadblast them, but that's kinda drastic and not that effective, I've tried a few already. Sure would appreciate any good tips on a soaking solution! Thanks!